The live-attenuated yellow fever virus (YFV) vaccine 17D, developed by serial passaging in vitro of the virulent Asibi strain, is one of the most successful vaccines, providing an essentially life-long protection from disease caused by infection with wildtype YFV. While the vaccine strain is markedly less viscerotropic and neurotropic than virulent strains, the mechanism of attenuation remains completely unknown despite more than 70 years of successful 17D vaccine use and knowledge of the sequences of both virus strains. New preliminary studies using NRG mice (non-obese diabetic mice harboring the Rag1null and IL2r?null mutations), which lack B, T and NK cells, but retain a functional innate immune system, demonstrate that infection with Asibi results in 100% lethality, while 17D infection is non-lethal. Moreover, infection of primary fetal hepatocytes or iHEPs with Asibi results in persistent replication, while 17D is cleared. We hypothesize that Asibi virulence is related to the ability to counteract (or not induce) anti-YFV innate immunity, which is lost in 17D leading to its attenuation. Moreover, we posit that otherwise healthy individuals suffering from severe vaccine adverse reactions harbor genetic mutations in components of innate immune pathway(s) that normally control 17D, but are counteracted by Asibi and other virulent strains. Using molecular virologic and genomic approaches in cell-based and small animal model-based systems, as well studies on human samples, this project will 1) identify the differences between Asibi and 17D in their interaction with host innate immune pathways, 2) map the genetic determinants responsible for 17D attenuation, and 3) characterize the host genetic determinants associated with serious adverse events to 17D vaccination. The work will provide information on responses to the YFV 17D vaccine, the mechanism of vaccine attenuation, and may uncover novel innate immune antagonism mechanisms. Knowledge of genetic defects predisposing to adverse events will allow pre-vaccine testing to identify at-risk individuals. All together the wor will support the development of safe and effective vaccines for YFV and other flaviviruses.